This invention relates to feeding ink ribbons and in particular to detection of faults in feeding of the ribbon.
Ink ribbons for printing on a print receiving medium are commonly fed from a supply spool past a printing head where printing is effected and then the used ribbon is wound onto a take-up spool. Means are often provided to detect when the end of the ribbon is approaching so as to provide an indication that the supply of ribbon is exhausted. If the ink ribbon supply is exhausted, continued operation of the print head will not result in printing on the print receiving medium. For many printing applications interruption of printing of a block of printing, such as printing of a sheet, is an inconvenience but otherwise is of little importance and at worst in the event of such an occurrence, after replenishment of the ink ribbon supply, the sheet can be re-printed.
However in a franking machine for printing franking impressions on mail items, it is required that when a printing cycle has been initiated sufficient ribbon is available for use to print the entire franking impression. Otherwise, if the ribbon should become exhausted during printing of the franking impression, only a partial franking impression would be printed and this would be unacceptable as a franking of the mail item while accounting for the value of the postage charge would have been completed prior to commencement of printing.
If the path of travel of new ink ribbon from the supply to the print head is of sufficient length a sensor to detect the end of the ribbon can be positioned a sufficient distance from the print head to provide a signal indicating that the supply of ribbon is exhausted while a length of ribbon sufficient to print an entire franking impression is contained in the path of travel between the sensor and the print head. However in order for the franking machine to be compact and of convenient overall size it is required that the ribbon supply spool is positioned closely adjacent the print head. As a result the length of ribbon required to print a franking impression may be wrapped a number of times round the supply spool and accordingly sensing of the end of the ribbon is difficult.